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4. Objectivity, Publicity, and Intersubjectivity: Evidence as Neutral Arbiter What is creditable … is not the mere belief in this or that, but the having arrived at it by a process which, had the evidence been different, would have carried one with equal readiness to a contrary belief. —Blanshard, Reason and Belief It is natural to suppose that the concept of evidence is intimately related to the cognitive desideratum of objectivity. According to this line of thought, individuals and institutions are objective to the extent that they allow their views about what is the case or what ought to be done to be guided by the evidence, as opposed to (say) the typically distorting influences of ideological dogma, prejudice in favor of one's kin, or texts whose claim to authority is exhausted by their being venerated by tradition. To the extent that individuals and institutions are objective in this sense, we should expect their views to increasingly converge over time: as shared evidence accumulates, consensus tends to emerge with respect to formerly disputed questions. Objective inquiry is evidence-driven inquiry, which makes for intersubjective agreement among inquirers.http://plato.stanford.edu/entries/evidence/notes.html#26 26 Thus, it is widely thought that the reason why the natural sciences exhibit a degree of consensus that is conspicuously absent from many others fields is that the former are evidence-driven—and therefore, objective—in ways that the latter are not. According to this picture, a central function of evidence is to serve as a neutral arbiter among rival theories and their adherents. Whatever disagreements might exist at the level of theory, if those who disagree are objective, then the persistence of their disagreement is an inherently fragile matter, for it is always hostage to the emergence of evidence which decisively resolves the dispute in one direction or the other. Our ability to arrive at consensus in such circumstances is thus constrained only by our resourcefulness and ingenuity in generating such evidence (e.g., by designing and executing crucial experiments) and by the generosity of the world in offering it up. The slogan ‘the priority of evidence to theory’ has sometimes been employed in an attempt to capture this general theme. However, this slogan has itself been used in a number of importantly different ways that it is worth pausing to distinguish. First, the claim that ‘evidence is prior to theory’ might suggest a simple model of scientific method that is often associated (whether fairly or unfairly) with Francis Bacon. According to the model in question, the collection of evidence is temporally prior to the formulation of any theory or theories of the relevant domain. That is, the first stage in any properly-conducted scientific inquiry consists in gathering and classifying a large amount of data; crucially, this process is in no way guided or aided by considerations of theory. Only after a large amount of data has been gathered and classified does the scientist first attempt to formulate some theory or theories of the domain in question. On this model, then, evidence is prior to theory within the context of discovery. This model—which Hempel (1966) dubbed ‘the narrow inductivist account of scientific inquiry’—is now universally rejected by philosophers. For it is now appreciated that, at any given time, which theories are accepted—or more weakly, which theories are taken to be plausible hypotheses—typically plays a crucial role in guiding the subsequent search for evidence which bears on those theories. Thus, a crucial experiment might be performed to decide between two rival theories T1 and T2; once performed, the outcome of that experiment constitutes an expansion in the total evidence which is subsequently available to the relevant scientific community. If, however, the two leading contenders had been theories T1 and T3, a different crucial experiment would have been performed, which would have (typically) resulted in a different expansion in the total evidence. The point that the formulation of hypotheses is often temporally prior to the collection of evidence which bears on their truth (and that this priority is no accident) is perhaps most immediately apparent on Popper's falsificationist model of science (1959), according to which exemplary scientific practice consists in repeatedly attempting to falsify whichever theory is presently most favored by the relevant scientific community. But it is no less true on other, less radical models of science, which (unlike Popper's) allow a role for confirming evidence as well as disconfirming evidence. As Hempel puts the point, In sum, the maxim that data should be gathered without guidance by antecedent hypotheses about the connections among the facts under study is self-defeating, and it is certainly not followed in scientific inquiry. On the contrary, tentative hypotheses are needed to give direction to a scientific investigation. Such hypotheses determine, among other things, what data should be collected at a given point in a scientific investigation (1966, p. 13).http://plato.stanford.edu/entries/evidence/notes.html#27 27 A second, quite different, sense of ‘priority’ in which evidence has sometimes been held to be prior to theory is that of semantic priority. According to this view, the meanings of hypotheses that involve ‘theoretical terms’ (e.g., ‘electron’) depend upon the connections between such hypotheses and that which would count as evidence for their truth—typically on such accounts, the observations that would confirm them. The view that (observational) evidence is semantically prior to theories was central to the logical positivist conception of science. On this picture, meaning flows upward from the level of observation; a given theory is imbued with whatever meaning it has in virtue of standing in certain relations to the observational level, which constitutes the original locus of meaning. The picture was gradually abandoned, however, in the face of repeated failures to carry through the kind of theoretical reductions that the picture seemed to demand, as well as appreciation of the point, forcefully emphasized by Putnamhttp://plato.stanford.edu/entries/evidence/notes.html#28 28 and others, that the meanings of theoretical terms do not seem to change as our views about what counts as confirming evidence for hypotheses in which they occur evolve. A third and final sense of ‘priority’ in which evidence has often been thought to be prior to theory is that of epistemic priority. On this view, it is not that the task of evidence gathering either is or ought to be performed earlier in time than the task of formulating theories; nor is semantic priority at issue. Rather, the thought is that theories depend for their justification on standing in certain relations to evidence (understood here, once again, as that which is observed), but that observations do not themselves depend upon theories for their own justification. That is: (observational) evidence is prior to theory within the context of justification. This, perhaps, is the interpretation of ‘evidence is prior to theory’ on which the slogan enjoys its greatest plausibility. For it seems that our justification for believing any presently-accepted theory of natural or social science typically does depend upon suitable observations having been made, but that, on the other hand, one can be justified in taking oneself to have observed that such-and-such is the case even if there is at present no available theory as to why such-and-such is the case (and indeed, even if such-and-such's being the case is unexpected or unlikely given the theories that one presently accepts). To the extent that such a justificational asymmetry exists, there would seem to be some truth to the idea that evidence is epistemically prior to theory.http://plato.stanford.edu/entries/evidence/notes.html#29 29 And this sort of priority might seem to be exactly what is required if evidence is to play the role of neutral arbiter among those who come to the table with different theoretical commitments. The idea of evidence as a kind of ultimate court of appeal, uniquely qualified to generate agreement among those who hold rival theories, is a highly plausible one. Nevertheless, complications with this simple picture—some more serious than others—abound. Above, we took note of the widely-held view according to which the bearing of a given piece of evidence on a given hypothesis depends on considerations of background theory. Thus, two individuals who hold different background theories might disagree about how strongly a particular piece of evidence confirms a given theory, or indeed, about whether the evidence confirms the theory at all. Of course, if the question of who has the superior background theory is itself susceptible to rational adjudication, then this possibility need pose no deep threat to objectivity. Often enough, this will be the case. Suppose, for example, that you treat the fact that (i) the patient has Koplik spots on her skin as evidence that (ii) the patient has measles while I do not, simply because you know that Koplik spots are typically an effect of measles while I am ignorant of this fact. Here, that you treat (i) as evidence for (ii) while I do not is attributable to the straightforward superiority of your background theory to mine: you possess a crucial piece of medical knowledge that I lack. Presumably, if I were to acquire the relevant bit of medical knowledge, then I too would treat (i) as confirming evidence for (ii). The possibility that those who are relatively ill-informed might differ from those who are better informed in the way that they respond to evidence does not itself cast doubt on the capacity of evidence to play the role of neutral arbiter among theories; clearly, what is needed in such cases is for the worse informed party to become privy to those facts of which they are presently ignorant. However, a recurrent motif in twentieth century philosophy of science is that the bearing of evidence on theory is mediated by factors that might vary between individuals in ways that do not admit of such rational adjudication. Imagine two eminent scientists, both of whom are thoroughly acquainted with all of the available evidence which bears on some theory. One believes the theory, the other believes some different, incompatible theory instead. Must one of the two scientists be making a mistake about what their shared evidence supports? Perhaps the most natural answer to this question, at least at first blush, is ‘Yes’. A fair amount of twentieth century philosophy of science shied away from this natural answer, however, at least in part because of a growing awareness of just how often eminent, fully-informed and seemingly rational scientists have disagreed in the history of science.http://plato.stanford.edu/entries/evidence/notes.html#30 30 Thus, Thomas Kuhn (1962) would maintain that both scientists might be perfectly rational in holding incompatible theories, inasmuch as rationality is relative to a paradigm, and the two scientists might be operating within different paradigms. Similarly, Carnap (1952) would maintain that both scientists might be rational because rationality is relative to an inductive method or confirmation function, and the two scientists might be employing different inductive methods or confirmation functions. As we have noted, for the contemporary Bayesian, how it is reasonable to respond to a given body of evidence depends upon one's prior probability distribution: the two scientists might thus both be rational in virtue of possessing different prior probability distributions. There are, of course, important differences among these accounts of rationality. But notice that they each possess the same structure: what it is reasonable for one to believe depends not only on one's total evidence but also on some further feature F (one's prior probability distribution, paradigm, inductive method.). Because this further feature F can vary between different individuals, even quite different responses to a given body of evidence might be equally reasonable. On such views, the bearing of a given body of evidence on a given theory becomes a highly relativized matter. For this reason, the capacity of evidence to generate agreement among even impeccably rational individuals is in principle subject to significant limitations. Why is relativity to a prior probability distribution (paradigm, inductive method) any more threatening to the idea of evidence as neutral arbiter than the previously mentioned relativity to background theory? The difference is this: when I fail to treat Koplik spots as evidence of measles, there is a clear and straightforward sense in which my background theory is inferior to yours. By contrast, proponents of the views presently under consideration typically insist that a relatively wide range of prior probability distributions (confirmation functions, inductive methods, paradigms) might beequally good. For this reason, such views are often criticized on the grounds that they turn the relationship between evidence and theory into an overly subjective affair. Proponents of such views are not wholly without resources for attempting to alleviate this concern. In particular, many subjective Bayesians place great weight on a phenomenon known as the ‘swamping’ or ‘washing out’ of the priors. Here, the idea is that even individuals who begin with quite different prior probabilities will tend to converge in their views given subsequent exposure to a sufficiently extensive body of common evidence. However, the significance of the relevant convergence results is highly controversial.http://plato.stanford.edu/entries/evidence/notes.html#31 31 According to the accounts briefly canvassed here, two individuals of impeccable rationality might radically disagree about the bearing of a particular piece of evidence E on a given hypothesis H. Even in such cases, the disputants are not wholly without common ground. For they at least agree about the characterization of the evidence E: their disagreement concerns rather the probative force of Ewith respect to H. An even more radical challenge to the capacity of evidence to serve as neutral arbiter among rival theories concerns the alleged theory-ladenness of observation. According to proponents of the doctrine of theory-ladenness, in cases of fundamental theoretical dispute, there will typically be no theoretically-neutral characterization of the evidence available. Rather, adherents of rival theories will irremediably differ as to the appropriate description of the data itself.http://plato.stanford.edu/entries/evidence/notes.html#32 32 The doctrine of theory-ladenness is perhaps best appreciated when it is viewed against the background of the positivist tradition to which it was in large part a reaction. For the positivists, evidence was both epistemically and semantically prior to theory. Moreover, central to the positivist conception of science as a paradigm of rationality and objectivity is the idea that its disputes admitted of rational adjudication by appeal to evidence that could be appreciated by both sides. For these reasons, the positivists often insisted that the fundamental units of evidential significance—observation statements or ‘protocol sentences’—should employ only vocabulary that is within the idiolect of any minimally competent speaker of the relevant language. Ideally then, observation statements should be comprehensible to and verifiable by individuals who possess no specialized knowledge or terminological sophistication. Thus, Carnap recommended ‘blue’ and ‘hard’ as exemplary predicates for observation sentences.http://plato.stanford.edu/entries/evidence/notes.html#33 33 As against this, Hanson insisted that … the layman simply cannot see what the physicist sees … when the physicist looks at an X-ray tube, he sees the instrument in terms of electrical circuit theory, thermodynamic theory, the theories of metal and glass structure, thermionic emission, optical transmission, refraction, diffraction, atomic theory, quantum theory and special relativity (1961:19). But what holds for the physicist and the layman holds also for two physicists with sufficiently different theoretical commitments. Thus To say that Tycho and Kepler, Simplicius and Galileo, Hooke and Newton, Priestly and Lavoisier, Soddy and Einstein, De Broglie and Born, Heisenberg and Bohm all make the same observations but use them differently is too easy. This parallels the too-easy epistemological doctrine that all normal observers see the same things in x, but interpret them differently. It does not explain controversy in research science (1961:13). The extent to which observation is theory-laden remains a contested matter.http://plato.stanford.edu/entries/evidence/notes.html#34 34 To the extent that evidence which bears on a theory does not admit of a neutral characterization among those with sufficiently different theoretical commitments, this threatens to limit the ability of such evidence to successfully discharge the role of neutral arbiter in such cases. Still, whatever concessions to the proponents of theory-ladenness might be in order, the significance of the doctrine that they defend should not be overstated. For in any case, it seems undeniable that theories sometimes are discredited by the emergence of evidence which is taken to undermine them—even in the eyes of their former proponents.http://plato.stanford.edu/entries/evidence/notes.html#35 35 Let's assume then that evidence sometimes does successfully discharge the function of neutral arbiter among theories and is that which secures intersubjective agreement among inquirers. What must evidence be like, in order for it to play this role? That is, given that evidence sometimes underwrites intersubjective agreement, what constraints does this place on answers to the question: what sorts of things are eligible to count as evidence? Above, we noted that the traditional epistemological demand that one's evidence consist of that to which one has immediate and unproblematic access—and indeed, that one's evidence must be such that one can appreciate it (at least it in principle) even when one is in the most dire of epistemic predicaments—has often encouraged a phenomenal conception of evidence, according to which one's evidence is limited to one's experiences or sense data. On this picture, evidence consists of essentially private mental states, accessible only to the relevant subject. This picture of evidence stands in no small measure of tension with the idea that a central function of evidence is to serve as a neutral arbiter among competing views. For it is natural to think that the ability of evidence to play this latter role depends crucially on its having an essentially public character, i.e., that it is the sort of thing which can be grasped and appreciated by multiple individuals. Here, the most natural contenders would seem to be physical objects and the states of affairs and events in which they participate, since it is such entities that are characteristically accessible to multiple observers. (I ask what evidence there is for your diagnosis that the patient suffers from measles; in response, you might simply point to or demonstrate the lesions on her skin.) On the other hand, to the extent that one's evidence consists of essentially private states there would seem to be no possibility of sharing one's evidence with others. But it is precisely the possibility of sharing relevant evidence which is naturally thought to secure the objectivity of science. Indeed, it has often been held that inasmuch as the objectivity of science is underwritten by the fact that science is evidence driven, it is the public character of scientific evidence which is crucial. On this view, it is a central methodological norm of science to eschew as inadmissible (e.g.) any alleged episodes of incommunicable insight in considering whether to accept or reject a claim. The theme of the essentially public character of scientific evidence has been prominent from the earliest days of modern science—it was championed, for example, by Robert Boyle, founder of the Royal Society, who insisted that the ‘witnessing’ of experiments was to be a collective acthttp://plato.stanford.edu/entries/evidence/notes.html#36 36—and has remained so up until the present day. Unsurprisingly, the theme was taken up by philosophers of science with gusto. Thus, Hempel required that ‘all statements of empirical science be capable of test by reference to evidence which is public, i.e., evidence which can be secured by different observers and does not depend essentially on the observer’ (1952: 22). The idea was echoed by other leading positivists (see, e.g., Feigl 1953) as well as by Popper (1959). More recently, in the course of reviewing ‘some of the things objectivity, and specifically scientific objectivity, has been thought to involve’, Peter Railton singles out the idea that … objective inquiry uses procedures that are intersubjective and independent of particular individuals and circumstances—for example … it makes no essential use of introspective or subjectively privileged evidence in theory assessment (1985: 764). In short, it is not simply that the conception of evidence gleaned from exemplary scientific practice differs from the conception of evidence that traditional epistemology seemed to demand (as would be the case if, for example, one was somewhat more inclusive than the other). Rather, what seemed to be one of the characteristic features of scientific evidence, viz. its potential publicity, is the exactcontrary of one of the characteristic features of evidence as construed by much traditional epistemology, viz. its privacy. Such a tension was bound to generate a certain amount of dissonance. One historically noteworthy manifestation of this dissonance was the protracted debate within the positivist tradition over the nature of ‘protocol sentences’. Here, Carnap's odyssey is perhaps the most illuminating. Early in his philosophical career, Carnap, under the influence of Russell and Ernst Mach (and through them, the tradition of classical empiricism) took sense data as the ultimate evidence for all of our empirical knowledge. (Indeed, his early work Der Logischer Aufbau der Welt(1928) is largely devoted to the Russellian project of ‘constructing’ the external world out of sense data.) During this period, Carnap maintained that the protocol sentences that serve as the ‘confirmation basis’ for scientific theories refer to sense data or (to be more precise) ‘the sensing of sense data’. However, under the influence of both Popper and Otto Neurath, Carnap ultimately abandoned this view of protocol sentences in favor of one on which such sentences refer not to private mental events but rather to public objects and states of affairs. As recounted in his philosophical autobiography, the primary motivation for this change of heart was the growing conviction that the conception of evidence as private mental states rendered it inadequate to ground the intersubjectivity and objectivity of science.http://plato.stanford.edu/entries/evidence/notes.html#37 37 In this area as in others, the evolution of Carnap's own views was crucial to the evolution of the views of the Vienna Circle as a whole. The episode was well-recounted years later by Ayer: At the outset … the prevailing view was that these observation statements referred to the subject's introspectible or sensory experiences … It was held, following Russell and ultimately Berkeley, that perceiving physical objects was to be analyzed in terms of having sensations, or as Russell put it, of sensing sense data. Though physical objects might be publicly accessible, sense data were taken to be private. There could be no question of our literally sharing one another's sense data, any more than we can literally share one another's thoughts or images or feelings … This conception of elementary statements was exposed to attack on various grounds.… The most serious difficulty lay in the privacy of the objects to which the elementary statements were supposed to refer.… Because of such difficulties, Neurath, and subsequently Carnap, rejected the whole conception of elementary statements. They argued that if elementary statements were to serve as the basis for the intersubjective statements of science, they must themselves be intersubjective. They must refer, not to private incommunicable experiences, but to public physical events (1959: 17–20). Inasmuch as it is the distinctive function of protocol sentences to report one's evidence, a view about what sorts of contents a protocol sentence might have is in effect a view about what sorts of things can count as evidence. To abandon a view according to which the subject matter of any protocol sentence concerns the ‘private incommunicable experiences’ of some particular individual is in effect to abandon a version of the phenomenal conception of evidence. To adopt in its place a view according to which the subject matter of any protocol sentence concerns ‘public physical events’ is thus a particularly radical shift, inasmuch as this latter view seems to entail that only public physical events can count as evidence—and in particular, that experiences, formerly taken to exhaust the category of evidence, are ineligible to count as such.http://plato.stanford.edu/entries/evidence/notes.html#38 38 Underneath the positivists' changing views about the nature of protocol sentences, however, lay a fundamental assumption that they never questioned. The assumption in question is the following: that there is some general restriction on the subject matter of a sentence if that sentence is a potential statement of one's evidence. This assumption was explicitly rejected by Austin: It is not the case that the formulation of evidence is the function of any special sort of sentence.… In general, any kind of statement could state evidence for any other kind, if the circumstances were appropriate.… (1962: 116; emphases his). According to the alternative picture sketched by Austin, there are, indeed, circumstances in which one's having knowledge that things are a certain way depends upon one's having ‘phenomenal’ evidence provided by the fact that is how things look, seem, or appear. Traditional epistemology errs, however, in thinking that this is the general case. Indeed, there are circumstances in which the fact that things are a certain way constitutes one's evidence for judgements about how they look, seem or appear.http://plato.stanford.edu/entries/evidence/notes.html#39 39 Inasmuch as he denies that there are any general restrictions on the subject matter of an evidence statement, Austin's way of thinking about evidence is considerably more liberal or inclusive than that of much of the tradition. In this respect, his account of evidence resembles Williamson's (2000) later theory. As we have seen, Williamson holds that one's evidence consists of everything that one knows. In particular, one's evidence is not limited to one's knowledge of one's experiences, nor is it limited to one's observational knowledge—one's evidence also includes any theoretical knowledge that one might possess (p. 190). On such liberalized views, although one's evidence is not limited to one's introspectively arrived at knowledge of one's experiences, it includes everything that one knows about one's experiences on the basis of introspection. In this respect, such views are incompatible not only with the phenomenal conception of evidence but also with views that would rule out the objects of introspection as evidence on the grounds that the objects of introspection lack the objectivity and publicity that is characteristic of genuine evidence. However, it is dubious that any view on which evidence plays a role in justifying belief can consistently observe a constraint which would preclude the objects of introspection from counting as genuine evidence. Goldman (1997) argues that any such constraint is inconsistent with the introspectionist methodology employed in various areas of contemporary cognitive science and that this undercuts ‘the traditional view … that scientific evidence can be produced only by intersubjective methods that can be used by different investigators and will produce agreement’ (p. 95). Reflection on examples drawn from more homely contexts also casts doubt on the idea that all genuine evidence is in principle accessible to multiple individuals. When one has a headache, one is typically justified in believing that one has a headache. While others might have evidence that one has a headache—evidence afforded, perhaps, by one's testimony, or by one's non-linguistic behavior—it is implausible that whatever evidence others possess is identical with that which justifies one's own belief that one has a headache. Indeed, it seems dubious that others could have one's evidence, given that others cannot literally share one's headache. Here then we see another context in which theoretical demands are placed on the concept of evidence that seem to pull in different directions. On the one hand, it is thought central to the concept of evidence that evidence is by its very nature the kind of thing that can generate rational convergence of opinion in virtue of being shared by multiple individuals. This encourages the idea that any genuine piece of evidence can in principle be grasped by multiple individuals; anything which cannot be so grasped is either not genuine evidence or is at best a degenerate species thereof. On the other hand, evidence is taken to be that which justifies belief. And it seems that many of the beliefs which individuals hold about their own mental lives on the basis of introspection are justified by factors with respect to which they enjoy privileged access. Notably, the positivists' embrace of the idea that protocol sentences refer exclusively to publicly-observable physical objects and events was accompanied by an embrace of behaviorism in psychology.http://plato.stanford.edu/entries/evidence/notes.html#40 40 It is characteristic of behaviorism to denigrate the idea that the deliverances of introspection can constitute genuine evidence; on this combination of views then, the thesis that all evidence consists of that which can be shared by multiple observers is upheld. For those who reject behaviorism, however, the idea that at least some evidence does not meet this condition is a more difficult one to resist